The trust gave £318,876.66 to the Scottish Tories between 9 April 2001 and 28 February 2018, as well as direct donations to Jackson Carlaw MSP, Banff and Buchan MP, David Duguid, and Moray MP, Douglas Ross.

We revealed then SUAT was not registered properly with the Electoral Commission, had no clear address, legal structure or trustees, and was inconsistently described on numerous official returns.

Concerns over the so-called “dark money” trust were raised in Holyrood and Westminster and one trustee, Peter Duncan, a former Tory MP, resigned during the furore.

But 14 months later the Electoral Commission is yet to publish the results of its investigation into SUAT.

SUAT plays a significant role in funding Conservative campaigning in Scotland. Since June 2018, when concerns were first raised by The Ferret, SUAT has donated a further £34,208 to the Scottish Conservatives.

An STV investigation found that £29,000 from the trust accounted for a fifth of all party spending in the run-up to 2017’s snap election.

Jo Maugham QC, a barrister with the Good Law project said that 14 months seemed “an excessively long time” for the probe, and questioned whether the Electoral Commission was “really investigating at all.”

“Everyone appreciates that public authorities are constrained by resources, but if investigations take so long that they cross over into other democratic events, it undermines their whole purpose,” he added.

Campaign group Unlock Democracy urged the Electoral Commission to publish is investigation promptly, and stressed that a second investigation by the Westminster Parliamentary Standards Authority cannot get underway until the Electoral Commission publishes its results.

“It is a problem that the investigation has been slow to come to a conclusion, which also delays the UK Parliament’s probe into donation registration irregularities,” said Sarah Clark, a spokesperson for Unlock Democracy.

“This lacklustre response is typical of our political system, which is a playground for the rich. The system we have is rigged to allow access and influence to be purchased by the highest bidder, and those who hold political power are benefiting from the status quo,” she added.

“So long as dark money flows into the coffers of political parties and the pockets of politicians they will turn a blind eye and fail to take the action needed.”

Maugham is at the forefront of high-profile litigation at the Scottish courts that aims to ensure Prime Minister Boris Johnson complies with legislation forcing him to postpone the day the UK leaves the European Union. A final decision on the case is expected to be made by the Supreme Court next week.

The constitutional wrangling in the courts and in Westminster over brexit are widely believed to be raising the chances of a general election in the coming months.

An SNP spokesperson said: “With a general election looking likely, the public deserve to know who they are voting for and who funds the Tory party.

“It’s absolutely essential that the Electoral Commission cracks down on any suggestion of ‘dark money’ or murky donations, which could seriously undermine our democratic process.

“Transparency over party finances is absolutely vital in any healthy democracy.”

A spokesperson for the Electoral Commission said: “Our investigations are robust and evidence-based, and last as long as necessary for us to reach the correct outcome. We conclude cases as quickly as possible subject to those principles.”

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